Electrodeposition of selenium



Patented Aug. 18, 1953 ELECTRODEPOSITION F SELENTUM Mortimer 0. Bloom and Joshua. P. Levy, Boston,

Mass.,

assignors to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware No Drawing. Application June 10, 1948, Serial No. 32,275

The present invention relates to the manufacture of selenium rectifiers, selenium photocells and similar devices and is. more particularly concerned with improvements in that type of pro- ,cedure which involves the direct electrodeposition of selenium in the gray crystalline metallic state.

In connection with the manufacture of selenium rectifiers it is now known that selenium rectifier elements having excellent physical and electrical properties can be manufactured by the direct electrodeposition of gray crystalline metallic selenium upon an electrically conductive base element which is used as a cathode during the electrolysis of a suitable electrolyte, such as a solution of selenium dioxide. The essential features of the procedure for carrying out such direct electrodeposition are those described in the copending application of Von Hippel and Bloom, Serial No. 496,760, filed July 30, 1943, and per se,

form no part of the present invention. That procedurev makes it possible to produce a tenaciously adherent, completely continuous and substantially uniform layer of gray crystalline metallic selenium upon a suitable base element provided that the conditions under which the electrolysis is performed are appropriately controlled so as to avoid hydrogen evolution and/or the electrodeposition of amorphous red selenium and provided further that the materials constituting components of the electrolytic bath are suitably processed to remove undesired contaminants.

In carrying out the aforesaid procedure it was found at times that the electrodeposited selenium did, not have the requisite physical and electrical properties or that having such properties initial- 1y, it did not retain them. Further investigations have revealed that these erratic and non-uniform results were a function of certain impurities present, even though those impurities were present to only an extremely small extent. Impurities of an inorganic character can be eliminated by the known principles of analytical chemistry but organic impurities present a difficult and troublesome: problem which cannot be solved in such manner and it is particularly with respect to the .removal of organic impurities that the present invention is concerned. These organic impurities may be present due to traces of organic compounds in the materials used or may result from drag-in from the atmosphere or other fortuitous contamination.

In an effort to overcome the adverse influence of these contaminants which interfere with. the electrodeposition of a gray crystalline metallic selenium deposit or film having the requisite 7 Claims. (Cl. 20456) characteristics, various elaborate procedures have been developed or proposed. These include procedures such as the extreme refinement of the operating conditions, the use of expensive, highly purified reagents, working in an air-conditioned atmosphere and other extraordinary precautions. While these may satisfactorily accomplish the desired results, they not only materially increase the cost of manufacturing selenium rectifiers but they necessitate the use of highly skilled labor which is frequently unavailable.

It accordingly, one of the objects of this invention to provide a simple, easily carried out, relatively inexpensive procedure for rendering innocuous. those organic contaminants which become fortuitously introduced into, the electrolyte and which wouldinterfere with the electrodeposition of a selenium film or coating having the desired physical and electrical characteristics or which may reduce the efIic-iency of such electrodeposition.

Another object of the invention. resides in introducing into the electrolytic bath-or forming it therein situ-a body of material. which has noadverse effect upon the electrodeposition but which serves as an agent for the removal of the impurities.

Another object of the invention resides in procedure of the character specified which is adaptable to' continuous operation so that the electrolyte may be constantly maintained in a condition which yields the required character of electrodeposit even though extraneous materials such as organic contaminants become introduced during the electrolytic operation.

More specifically, an object of the invention is to introduce into contact with the electrolyte from which. the selenium deposit is to be or is being produced by electrolysis, an amount of metallic selenium in a state of sub-division which presents a considerable surface area to the electrolyte and which is capable of adsorbing the contaminants to render them innocuous and/or to facilitate their removal, said contact being preferably effected at a temperature substantially elevated above room temperature.

Other and further objects of the invention will be apparent or. pointed out hereinafter or will be understood and appreciated by those skilled in this art.

To facilitate a fuller and more complete understanding of the present invention. and. the manner of practicing the process, certain specific embodiments thereof are described below by way of example, it being understood that such are 3 illustrative only and do not constitute a limitation upon the invention:

Example I An electrolytic bath containing approximately 1225 grams of selenium dioxide dissolved in about 370 ml. of sulphuric acid of specific gravity 1.84 and about 370 m1. of water was made up in accordance with Example I of the aforesaid application, Serial No. 496,760, filed July 30, 1943, for use in electrolysis in the manner therein set forth. Using this bath for selenium electrodeposition Without especial precautions to avoid atmospheric contamination thereof, it was found after a period of use that the deposits obtained were unsatisfactory in terms of both electrical and physical properties. The bath was then brought. into contact with a number of pellets of the gray crystalline metallic allotropic form of selenium' at an elevated temperature, say about 100 0., and for a period of at least 12 and preferably 36 hours. These pellets were of such size and were used in such amount as to present a comparatively large surface of selenium to the bath and the adsorbable impurities therein. Inasmuch as the period of treatment necessary is an inverse function of the surface area of selenium in contact with the bath, it will be apparent that the period required to accomplish the desired results can be decreased by increasing the total area of selenium in contact with the solution.

It was found that, when selenium pellets or particles were thus brought into contact with the electrolyte, the organic impurities were rapidly rendered innocuous and were completely or substantially completely adsorbed onto the surfaces of the selenium pellets or particles.- This can be confirmed from the improvement which is found in the electrical characteristics of the electrodeposits, particularly with respect to the forward conductivity thereof, after the addition of the selenium pellets or particles. In addition, by testing the electrical characteristics of the electrodeposits, it can be determined when to add or change such selenium pellets or particles. Electrolysis carried out under the conditions of the above identified copending application, when using an electrolyte after treatment with selesurface area for adsorption. While no particular amount of selenium need be employed in a given case, the quantity should not be so great as to present mechanical interference to electrolysis nor should the fineness thereof be such as to give rise to difficulties in its removal by filtration after adsorption has occurred.

Example II Example I was repeated except that selenium was formed in situ by precipitation directly within the electrolytic bath as by an agent like sulphur or sulphur dioxide. The selenium so produced is in a finely divided state and not only has a large active surface but is in a highly active condition. Under such conditions the contaminants which are present in the electrolyte or which become fortuitously introduced thereinto are rapidly adsorbed on the selenium surfaces and rendered innocuous without adverse 4 effect upon the electrodeposition of the desired type of metallic selenium film or coating on the cathode.

Example III Example I was repeated except that the selenium employed in the electrolytic bath for adsorption purposes was in amorphous allotropic form instead of in the gray crystalline metallic form as in Example I. This type of selenium was also found to be effective for the present purposes when the conditions of operation favored transformation into the gray crystalline state.

7 Example IV Electrolytic baths which had become incapable of producing the desired selenium electrodeposits due to the presence of organic impurities were treated with selenium pellets or particles in accordance with the foregoing examples and were then again capable of effective use for selenium deposition.

Example V Three electrolytic baths (containing approximately 1225 grams of selenium dioxide dissolved in about 370 ml. of sulphuric acid of specific gravity 1.84 and about 370 ml. of water) were prepared as in Example I, using three different lots of selenium dioxide which spectroscopic analysis indicated to be of very high purity.

SPECTRO SC OPIC ANALYSIS (Parts per million) Lot #1 Lot #2 Lot #3 u 0. l u 0. 1 u 0.1 l u l u 1 0.3 0. 5 0.1 u 0.2 u 0. 2 u 0. 2 u 0.5 u 0. 5 u 0. 5 u 0.05 u 0. 05 0. 05 u l u 1 u l u 2 3 3 0. 4 0.5 0.3 u 1. 5 u l. 5 u 1.5 u l. 5 u 1. 5 u 1. 5 u 1. 5 u l. 5 u 1. 5

0. 6 0. 4 1. 5 u l. 5 u l. 5 u l. 5 u 5 11 5 u 5 u 1. 5 u 1. 5 u 1. 5 u 3 u 3 u 3 u=under. 1 Not detected.

It was observed that the baths prepared from lots #2 and #3 had a pink color (probably due to colloidal selenium) which disappeared (by coagulation and settling) after the bath had been heated at C. for some time, so that all the baths subsequently had the same appearance.

It was, however, observed that the character of the current-time curve obtained during electrolysis of lots #2 and #3 was markedly different from that obtained during electrolysis of lot #1 in the following respect. Using platinum electrodes, a voltage was initially applied sufiicient to produce a current density of 10 amps. per sq. ft. and this was maintained for about ten minutes, after which the voltage was raised to a value sufficient to produce a current density of 100 amps. per sq. ft.

A decrease of current density at constant voltage was exhibited by lots #2 and #3 after the current density had been raised to 100 amps. per sq. ft., which phenomenon will be called herein secondary polarization.

Milliamps. at 1.1 volts A. O.

Lot

It was found, moreover, that the electrolyte prepared from lots #2 and #3 tended to produce increasing secondary polarization with use, accompanied by pitted deposits aind shorted rectifiers.

The eifect of treatment with selenium pellets at 100 C. on the behavior of the electrolytes prepared from lots #2 and #3 is shown in the following table:

Milliamps. Electrolyte fromi at 1.1 v. Remarks 36 s; s d P 1 econ ary OIl- Lot #2 489 3 zation.

490 3 Selenium pellets added:

After 5 days 540 9.5 After 5 days 542 8.7 No secondary Pol- Aiter 6 days 556 15.8 arlzation. After 21 days 635 14.5

223 22 Lot #3 230 g gg 231 232 Selenium pellets added:

it a N 0 Secondary Pol- 244 arization.

Example VI A large number of organic compounds were added in small quantity to baths prepared as in Example I. Development of pink coloration, secondary polarization, low conductivity, and/or pitted deposits resulted in a very large number of cases.

Of the compounds thus tested, two namely, orthocresyl acetate and iso-amyl ether, were chosen as typical and were used for the following test.

To an electrolytic bath yielding satisfactory deposits, parts per million of ortho-cresyl acetate were added. A finely divided red precipitate of selenium dispersed throughout the bath was obtained due to oxidation of the organic compound with reduction of the selenium dioxide. It was found possible to remove this red selenium by repeated filtration through a very fine sintered glass filter and one-half of the bath was thus filtered, removing the selenium and leaving in solution the organic reaction products. The other half of the bath was allowed to remain at 100 CZ, without filtration, the finely divided precipitated selenium being thus kept in constant contact with the dissolved organic reaction products.

The test was then repeated using 15 p. p. m. of iso-amyl ether in a fresh sample of the electrolytic bath. Rectifiers were then prepared from 6 these baths as in Example V with the following results:

Milliamps. at r 1.1 v. A. 0. Remarks Before addition of organic com- 13.8 No Secondary Popound. larization. 4 days after addition of ortho- 2.5. Secondary Polarizcresyl acetate (filtered). ation.

4 days after addition of iso-amyl Short Do.

ether (filtered).

4 days after addition of orthol4 No Secondary Pocresyl acetate (unfiltered). larization.

4 clays after addition of iso-amyl I 19 Do.

ether (unfiltered).

Example VII To an electrolytic bath, prepared as in Example I, which had been badly contaminated with the volatile decomposition products of an oil bath so that it was dark yellowish brown in color, a considerable quantity of selenium was added in pellet form, and the bath was maintained at C.

Prior to addition of the selenium it was not found possible to obtain deposits of gray selenium free of the red variety. At the end of one week of treatment, the bath had been decolorized and it was found possible to obtain deposits of gray selenium free of the red variety. At the end of three weeks of treatment, normal deposits were obtained.

The foregoing is intended as illustrative and not as limitative since the invention is capable of various specific embodiments without departing from the spirit or principles hereof. While the above examples clearly demonstrate the value and utility of the invention other modifications may nevertheless be made within the purview of the invention which is rather of the scope defined by the appended claims.

We claim:

1. A process of regenerating, acidic aqueous selenium dioxide electrolytic bath contaminated with organic matter which comprises adding thereto elemental selenium in the form of particles in a quantity greater than the normal metallic selenium content of said bath, which 'particles present a substantial total surface to contact with said bath and leaving the added selenium in the bath at approximately 100 C. for a period of twelve hours to six days until it adsorbs said organic matter on the surfaces of said particles.

2. A process for preventing organic matter in acidic aqueous selenium dioxide electrolytic bath from adversely afiecting the plating qualities of said bath and the selenium deposits produced therefrom, which comprises adding to said bath particles of elemental selenium greater in size than those selenium particles in colloidal suspension in said bath, in a quantity greater than the normal metallic selenium content of said solution, but less than the quantity which presents mechanical interference to electrolysis, and adsorbing such organic matter on the surfaces of said added particles.

3. In a process in which gray crystalline metallic selenium is electrodeposited on a cathode from acidic aqueous solution of selenium dioxide, the steps which comprise adding particles of elemental selenium to such solution, said particles being added in a quantity greater than the normal metallic selenium content of said solution, said particles being greater in size than those in colloidal suspension in said solution, and

carryingout'electrodeposition' with the sotreated solution.

4. In a process in which gray crystalline metallic selenium is electrodeposited on a cathode from acidic aqueous solution of selenium dioxide, the steps for regenerating said solution which has become contaminated with organic matter which comprise adding elemental selenium in the 'form of particles of its gray crystalline metallic allotrope to such solution, said particles added being in a quantity greater than the normal mewhich comprise adding elemental selenium in the form of particles of its amorphous allotrope to such solution in a quantity greater than the normal metallic selenium content of said solution, but less than the quantity which presents mechanical interference to electrolysis, under conditions favoring the transformation of the added selenium to the gray crystalline state, and carrying out selenium electrodeposition with the so-treated solution.

6. A process for maintaining acidic aqueous selenium dioxide solution in a condition in which gray metallic selenium can be electrodeposited therefrom even in the presence of organic contaminants which normally interfere with such electrodeposition, which comprises introducing elemental selenium into said solution in finely divided form in a quantity greater than the normal metallic selenium content of said solution, but less than the quantity which presents mechanical interference to electrolysis, said introduced selenium having an extensive aggregate surface on which such contaminants are capable of being adsorbed and maintaining a supply of such elemental selenium in such solution sufficient to render innocuous fortuitously present amounts of such contaminants.

7. A process for treating an acidic aqueous selenium dioxide electrolytic bath to render innocuous organic impurities therein, which comprises adding thereto a quantity of elemental selenium in the form of particles in a quantity greater than the normal metallic selenium content of said solution, but less than the quantity which presents mechanical interference to electrolysis, and which particles present a substantial total surface to contact with said bath, and maintaining the bath at approximately 100 C. and in contact with said particles for a period from twelve hours to six days until the effect of the impurities ceases.

MORTIMER C. BLOOM. JOSHUA P. LEVY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,103,017 Ellis July 7, 1914 2,248,092 Korpium July 8, 1941 2,322,348 Clark June 22, 1943 2,414,438 Bloom Jan. 21, 1947 2,568,780 Von Hippel et al. Sept. 25, 1951 OTHER REFERENCES Alexander, Colloid Chemistry, vol. 1 (1926), pp. 660-1.

Mellor, Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol, X (1930), pp. 702-3, 763, 850.

Sella, Chemical Abstracts, vol. 34 (1940), p. 2713. 

1. A PROCESS OF REGENERATING ACIDIC AQUEOUS SELENIUM DIOXIDE ELECTROLYTIC BATH CONTAMINATED WITH ORGANIC MATTER WHICH COMPRISES ADDING THERETO ELEMENTAL SELENIUM IN THE FORM OF PARTICLES IN A QUANTITY GREATER THAN THE NORMAL METALLIC SELENIUM CONTENT OF SAID BATH, WHICH PARTICLES PRESENT A SUBSTANTIAL TOTAL SURFACE TO CONTACT WITH SAID BATH AND LEAVING THE ADDED SELENIUM IN THE BATH AT APPROXIMATELY 100* C. FOR A PERIOD OF TWELVE HOURS TO SIX DAYS UNTIL IT ADSORBS SAID ORGANIC MATTER ON THE SURFACES OF SAID PARTICLES. 